The present invention relates to a compressive and electro-osmotic dehydrator of the filter press type, wherein liquid containing solids such as sludge or slurry of service water or sewage is dehydrated under pressure, and electro-osmotically to improve the dehydration. Particularly, the invention relates to improvements in the supports for the electrodes and diaphragms of the dehydrator.
Many types of prior art arrangements for electro-osmotic dehydration are described in U.S. Pat. No. 4,376,022 to Porta et al., while electro-osmotic dehydration with a filter press is described in U.S. Pat No. 3,773,640 to Burrage et al.
Compressive and electro-osmotic dehydrators of the filter-press type having press diaphragms are shown in Japanese provisional patent publications No. 62-125810, No. 62-125811 and No. 62-125812 of Shinko Pfaudler Co., Ltd. (now Shinko Pantec Co., Ltd.), all laid open on June 8, 1987.
FIGS. 1a-1c herein show steps of operation of this type of conventional dehydrator, which includes press plates 2a and 2b and a pair of filter cloths 3 between them. The plate 2a has a diaphragm 4 supporting an electrode plate 5a. The other plate 2b has an electrode plate 5b fixed to it.
In the first step shown in FIG. 1a, the press plates 2a and 2b are closed to form a sealed space between them. A liquid containing solids is pumped through passages 6 and squeezed between the cloths 3. An amount of liquid filtrate permeates through the cloths 3 and drains through small holes in the electrodes 5a and 5b and through passages 7. This produces a cake C of sludge and constitutes primary dehydration.
In the second step shown in FIG. 1b, compressed air is supplied to the space between the plate 2a and the diaphragm 4, which swells the diaphragm and compresses the sludge cake C. This further dehydrates the cake down to a water content of 80-85%, and constitutes secondary dehydration.
In a later stage of this step, DC voltage is applied, for example, at 40 volts for 15 minutes between the electrodes 5a and 5b. This causes electro-osmosis to further promote the dehydration, and constitutes tertiary dehydration. As a result, the water content is reduced to about 50% and the cake volume is greatly reduced.
In the last step shown in FIG. 1c, the press plates 2a and 2b are opened and the cloths 3 are lowered in order to remove the finally dehydrated cake C.
The electrodes 5a and 5b, which are expensive, are easily damaged electrochemically, while the electrode 5a is also subjected to strong mechanical actions of the diaphragm 4 which supports it. Conventionally, the electrodes have been made of metal plates, sintered carbon plates or the like.
If the electrode 5a fixed to the diaphragm is a metal plate, it can be constructed advantageously in terms of bending strength, but this is electrochemically disadvantageous.
If the electrode 5a comprises a sintered carbon plate, it must be made of special carbonic material advantageous for electro-osmosis. In general, such material is brittle and does not bend. Consequently, for a large plate size, the electrode of this material must be divided into segments so that it is not broken by high pressure. The provision of divided electrodes on a flexible diaphragm would involve difficulty in uniform voltage application to the electrode segments, complexity in construction for their individual supports and reinforcements, difficulty in providing liquid passages through the electrode segments, and ready exposure to the influences of electrode reaction.